Make LabHub is_room_member a private function of the module

[jayvdb]

LabHub.is_room_member is unnecessarily a static method of the class.

It can be moved to a private function _is_room_member at the module level, and then the tests can be improved to remove the need to do special plugin mocking.

See #572 (comment) for context

[srivama]

Assign this to me.

[kumar1202]

@jayvdb Can you assign it to me, if no one is working on it.

[srivama]

Ah yes! I was working on this issue earlier, but then it skipped my mind after the exams break. Feel free to work on it once you’re assigned 🙂